@TheFortold, right-click on the 0AD icon, and choose "show package content"
contents → resources → data → mods (to place a mod)
Most of the interesting stuff is in: mods → public → public
There are a lot of hidden files on a macbook, and sometimes you'll have to use the ~/Library (which itself is also hidden...) to follow certain file-paths.
MacBook makes finding specific files aggravatingly difficult/annoying if you're unfamiliar with this kind of stuff, like I am.. Finding save-games or user-created maps becomes a whole hullabaloo because of it.